In the past, both permanent and electromagnets have been employed in a variety of devices used in factories and other environments. Devices that require magnetic energy to be switched on and off generally employ electromagnets because the magnetic field produced by permanent magnets cannot be switched on and off. As a result, lifting devices, clamping devices, and other devices that require large magnetic forces to attract or in some other manner selectively interact with a ferromagnetic element employ electromagnets. As a general rule, permanent magnets are not employed in detachable magnetic devices, e.g., lifters and clamps, that require large magnetic forces because of the difficulty in detaching such devices, i.e., removing a lifter from a ferromagnetic part or separating the two elements of a magnetic clamp. Also, as a general rule, permanent magnets have not been used in high force generating devices that employ magnetic energy, such as riveters, because of the difficulty in controlling the interaction of the magnetic field with another element, e.g., the hammer of a riveter. As a result, contemporary riveters that employ magnetic energy are electromagnetic in nature.
While electromagnets are usable in factories and many other environments, they have a number of disadvantages in some environments. For example, electromagnets are undesirable in environments where potentially explosive gases are present because of the possibility that an arc will occur and ignite the explosive gases. Further, high-power electromagnets designed for use in factories require high voltage and/or large current sources, which can be dangerous. Electromagnets also tend to be bulky due to their inclusion of a relatively large coil wrapped around a core, usually formed of a ferromagnetic material. Further, electromagnets may exhibit substantial residual amounts of magnetism even when switched off, which may be undesirable in some environments.
While permanent magnets avoid some of the disadvantages of electromagnets, they have other disadvantages. As noted above, permanent magnets cannot be switched on and off. As a result, large mechanical forces are required to move strong permanent magnets toward or away from a part, or the part away from the magnet, in order to detach the permanent magnet from the part. The inability to switch permanent magnets on and off has, as noted above, severely restricted the use of such magnets, particularly high-power permanent magnets. Permanent magnets have not found use where high clamping or repulsive forces are required because of their inability to be turned on and off. As a general rule, electromagnets have generally been used in devices requiring switchable high magnetic clamping forces.
One exception is described in U.S. patent application Ser. No. 08/738,993, and titled "High Temperature Superconductor Magnetic Clamps" by D. F. Garrigus et al. This patent application describes switchable magnetic clamps that incorporate superconductor magnets. The clamp is switched on and off by controlling temperature of the superconductor magnets. Because superconductor magnets become superconducting at extremely low temperatures, the magnetic clamps described in this patent application require a complex and, thus, expensive temperature control system.
The present invention is generally directed to providing switchable magnetic devices suitable for use in a factory or other environment where the ambient temperature is approximately room temperature (70.degree. F.) that overcome the foregoing disadvantages. While directed to providing switchable permanent magnetic devices that have the capability of being switched on and off, the invention can also be used with electromagnets. As will be better understood from the following description, in addition to being usefully employed in lifters, clamps, and riveters, switchable magnetic devices formed in accordance with the invention can also be usefully employed in a variety of other devices. Further, while ideally suited for use in magnetic devices intended to operate in a room temperature environment, the invention can also be used in devices intended to operate in other, particularly low-temperature, environments, such as the environment in space.